The lithium secondary battery formed by using a material capable of absorbing and discharging lithium ions is capable of restraining precipitation of dendrite to a higher extent than a lithium battery in which the negative electrode is formed by using metallic lithium. Therefore the lithium secondary battery has been supplied to the market as a battery having enhanced safety. In recent years, the development of the lithium secondary battery is advanced for industrial use including a case in which the lithium secondary battery is mounted on a vehicle and a case in which it is used as a stationary power source. It is a big problem to allow the lithium secondary battery to have a high output (in charging and discharging it at high current) and a long life, even though it is repeatingly charged and discharged at high current.
To overcome this problem, there have been improvements including an increase in the capacity of a positive electrode material composed of a lithium metal oxide and in the capacity of the negative electrode material composed of a carbon-based material, a material containing a titanium oxide or an alloy-based material to allow high current to flow through the lithium secondary battery. The diameters of active substance particles are decreased to increase the specific surface area of the active substance and in addition the electrodes are so designed as to increase the areas thereof so that the current density load of the lithium secondary battery can be decreased.
The above-described devices have improved the performance of the lithium secondary battery in allowing the lithium secondary battery to be charged and discharged at high current, but were insufficient as a measure for prolonging the life of the lithium secondary battery. Therefore the substitution mixing ratio of metal elements of lithium metal oxides used to form the positive electrode and substitution of doped metals have been investigated. There has been proposed an additive devised to prevent a resistance film from being generated by the decomposition of an electrolytic solution at the negative electrode composed of a carbon-based material. To improve the performance of the negative electrode composed of an alloy-based material having a semiconductor property, there has been also proposed an alloy composition, the addition of a conductive material, and a binding agent devised to restrain the volume expansion of an alloy. For example, the electrode of the secondary battery composed of the active substance powders, the electrode material formed from the carbon material and attaching to the surface of the active substance powders, and the fibrous conductive material combined with the conductive material is known (see patent document 1).
As a separator to be interposed between the cathode and the anode, a polyethylene film having a porosity of about 40% is mainly used. In addition, to improve high-temperature storage performance and output characteristics in the range from high temperatures to low temperatures, there is proposed the separator consisting of cellulose fibers heat-resistant and excellent in impregnation performance for a nonaqueous electrolyte (patent document 2).